Transcript phys_layer-1

CS3502,
Data and Computer Networks:
the physical layer-1
physical layer - purpose

To transmit bits, by encoding them onto signals; and to
receive the signals, interpreting them as bits
input: sequence of bit S, from DL or MAC layer
output: sequence of bit S’, to DL or MAC layer
ideally, S = S’
Physical layer definitions
 signal
1. a mechanism used to carry information over
time OR distance
2. a sign or gesture giving information
3. a sequence of electrical impulses or waves
4. electromagnetic encoding of data
Signals
 examples:
 physical
gesture, wave, hand signal
 flashes of light (eg, Morse code)
 sound: vary tone, loudness or duration
 flags
 smoke
 mirrors
 electrical voltages
transmission definitions
1. the action of conveying electrical or optical
signals from 1 point to 1 or more other points
in space
2. the process of sending information from 1
point to another
 functions
 path
necessary for transmission systems
for signal transfer (medium)
 transform signal to appropriate form (code)
 launch the signal (transmit)
 remove, receive or detect the signal (receive)
transmission properties
 functions
present in some transmission
systems (often helpful)
 compensate
for loss in media
 compensate
for distortion introduced by
media
 control
access to the media (ie, protect from
interference)
 monitor
and control quality of transmission
signal - can be modeled as
f(t)
t --> f(t) ... f(t) represents some physical quantity: voltage,
amplitude, frequency, etc.
signals
 functions/signals
may be either ---
 continous
 no
breaks in the graph
or
 discrete
 takes
only a finite or countable number of values
Q: can a function be both?
Q: must a function be one or the other?
examples of functions
f(t) = 1
f(t) = c
f(t) = Roof(t)
f(t) = Floor(t)
f(t) = t - Floor(t)
f(t) = sin(t)
 you
should be able to graph all of these
digital/ analog signals
 digital
signal
1. assumed to take on finite number of
values, AND
2. has meaning only at discrete points in
time.
 digital
signals represented by discrete
functions. (however this is an idealized
and somewhat unrealistic picture).
digital/analog signals
 analogy:
a partial likeness between 2 things
that are compared (Oxford Dict.)
 analog
signal:
1. a signal that is an analog of the quantity
being represented; eg, signal voltage
proportional to volume of sound
2. continuous range of values
3. continuous write time; always valued.
digital/analog signals
 digital
data: text, bits; discrete valued.
 analog
data: sound, vision; music, etc. continuous
valued.
Note: digital (analog) signals can transport both
digital and analog data, so 4 combinations
(DD,DA,AD,AA) possible
 some
media only propagate analog signals
efficiently, and sometimes more efficient to
digitize analog data
digital/ analog signals
 periodic
function -- cyclical in values (note
mathematical definition)
3
key properties of periodic signals:
 amplitude:
instantaneous value
 frequency:
cycles per second (hertz)
 phase:
 these
position within a cycle/period
quantities are varied, in order to use
the signal to carry information
digital/ analog signals
 key
fact: any signal can be represented as
a sum (possibly infinite) of periodic
functions. (Fourier analysis
mathematical picture)
 f(t)
=
(1/2)·k0
+n=1.inf an·sin(2··n·f·t) +
n=1.inf bn·cos(2··n·f·t)
t=0…Tf(t) ·sin(2··n·f·t) ·dt
 bn = 2/T t=0…Tf(t) ·cos(2··n·f·t) ·dt
 k0 = 2/T t=0…Tf(t) ·dt ; the average amplitude

an = 2/T
digital/ analog signals
 (Fourier
analysis graphical picture)
.5·sin(2··7·f·t)
Tuned to 7·f
1·sin(2··6·f·t)
Tuned to 6·f
5·sin(2··5·f·t)
Tuned to 5·f
1·sin(2··4·f·t)
Tuned to 4·f
2·sin(2··3·f·t)
Tuned to 3·f
f(t)
4·sin(2··2·f·t)
Tuned to 2·f
8·sin(2··1·f·t)
Tuned to 1·f
a1=8
transmission media
 transmission
medium: the physical
element through which signals must
pass, from transmitter to receiver
 examples: air, water, (outer) space, copper
wires, optical fiber
 two main categories: guided and
unguided
 propagation delays of signals in media
transmission terminology
 direct
link
 simplex
 half-duplex
 full
duplex
 spectrum
a signal
- range of frequencies making up
 bandwidth
frequencies
 examples
- width of the spectrum; range of
transmission terminology
 note
1: bandwidth key factor in determining data
rate;
 note 2: however do not confuse bandwidth (hertz)
and and data rate (bps)
 attenuation
 amplifier
 boosts
energy of analog signal; increases amplitude
 makes no distinction between noise and signal
 repeater
 receives,
interprets and repeats a digital signal
 adds little or no noise/distortion
transmission terminology
 repeater-amplifier
diagram comparison
modems, codecs, bauds, bits
 modem
(modulator-demodulator)
 translates
a digital signal (bit) into an analog signal,
for transmission as an analog signal; receives the
corresponding analog signal, and translates back into
digital (bit)
 purpose:
use analog medium for digital data/signals
 example:
PC modem, phone lines; TV cable modems
 techniques:
PSK, ASK, FSK and combinations.
modems, codecs, bauds, bits
 codec
(codec/decoder)
 converts
analog data into digital form (bits),
and the reverse.
 two main techniques: PCM, DM
 PCM
(pulse code modulation)
 absolute
values, based on sampling theorem;
(nearly) total information
 DM
(delta modulation)
 based
on differences; less accurate
modems, codecs, bauds, bits
 Baud
rate -maximum number of times per
second that the signal element can change
 Baud
- The unit in which the Baud rate is
measured
 incorrect
to say “9800 bauds per second.”
 thus,
the baud rate is the rate at which distinct
signal elements are sent.
 also
called “signaling rate”
 applies
to digital signals or analog signals
carrying digital data.
modems, codecs, bauds, bits
 diagram:
bauds and bits
Bit rate =
3
baud rate·log2(#of signal
levels)
Example:
Signal 2
levels 1
0
T
=
1/Baud Rate
measured in baud
A 9600 baud modem has a
baud rate of 9600 baud. If it
uses two signal levels is also
runs at 9600 bps.